To reduce pressure fluctuating width by setting the profiles of a driving side non-circular gear and a driven side non-circular gear so that a reciprocating member performs approximately uniform motion against the constant velocity rotation of the driving side non-circular gear in a predetermined range between the top dead center and the bottom dead center of the reciprocating member.

An input shaft is rotatively driven at a constant velocity by a prime mover such as an engine. The constant velocity rotation of the input shaft is transmitted to a crankshaft while fluctuating its rotational speed by engaging a driving side non-circular gear 44 with a driven side non-circular gear 46. A plunger 30 of first and second plunger pumps is reciprocated at a constant velocity accompanied by the rotation of the crankshaft in approximately the whole intermediate range except for the vicinity of a top dead center and a bottom dead center. Hereby, a discharge flow in the discharge port 22 of a double plunger pump device 10 is equalized approximately in the whole range except for the vicinity of the top dead center and the bottom dead center, and a flow velocity passed through an inlet valve 26 and a delivery valve 28 is equalized.